I'll make it simple... don't listen to them lol.
5N means that entire bottle has 5% nitrogen. Thats neither meaningful nor meaningless on its own.
How that 5%N is used in your watering mix, and then in turn how it used by the plant are whats important.
You can use a 20-0-0 on flower if needed, but the amount and dilution rates are where you would need to be cautious because of the concentration of the nutrient. With that said, dont use a 20-0-0 in flower lol, as most bottled mixes come with exactly what your plant needs.
I checked out the Humboldt secret line, and it is an A+B all the way through, and golden tree would be your "micro"... so in this instance, because AB is actually just a grow and bloom, you likely could cut back on your part A some if you wanted to. BUT you aren't showing signs of N issues so there's no need to do that.
If i were you, id follow their chart, using the lower side of their application rate ranges (says 5-9ml, id use 5, or 7-10, id use 7, etc). The reason being, is that underfeeding slightly is an easy fix, and will be just as noticeable as over feeding, which can cause serious damage fairly quickly. But otherwise, id still go by their recommended applications, and like you said add a PK booster (which id start around week 4 and then taper it out towards the end) to make up for where its lacking.
Give us more pictures without the blurple light on so we can see what she looks like